SPECTROSCOPY OF LYMAN BREAK GALAXIES IN THE HUBBLE DEEP FIELD

We report on the initial results of a spectroscopic investigation of g alaxies in the Hubble Deep Field which exhibit spectral discontinuitie s between the F450W and F300W passbands, indicative of the presence of the Lyman continuum break in the redshift range 2.4 less than or simi lar to z less than or similar to 3.4. We have employed color selection criteria similar to those we have used for selecting high redshift ga laxy candidates from ground-based images. We find that, as for the gro und-based color selection, the criteria are very successful in selecti ng high redshift objects. Of the 8 galaxies observed (selected from a list of 23 candidates with magnitudes equivalent to R less than or equ al to 25.3), 5 have confirmed redshifts in the range 2.59 less than or equal to z less than or equal to 3.22, with the remaining 3 being ind eterminate because of contamination from nearby brighter objects. As e xpected, the HST filter system is sensitive to a somewhat broader rang e of redshifts than our ground-based U-n G R filter system, and theref ore the surveyed volume per unit area on the sky is correspondingly la rger. The distribution of candidates on the plane of the sky is clearl y non-uniform, consistent with the available ground-based data on the high redshift galaxies. Most Lyman break objects in the Hubble Deep Fi eld exhibit a similar range of morphological properties to the z>3 gal axies we have previously identified in other fields, characterized by very compact cores (some with multiple components) with half-light rad ii of 0.2-0.3 arcseconds, often surrounded by more diffuse and asymmet ric ''halos.'' A few of the brighter HDF Lyman break galaxies, however , have particularly unusual morphologies. (C) 1996 American Astronomic al Society.